1. Field of the Invention
The present invention generally relates to a field of disk drives, particularly relates to a read channel applied to a perpendicular magnetic recording type of disk drive.
2. Description of the Related Art
Recently, in the field of the disk drive, a perpendicular magnetic recording type of disk storage apparatus which can improve recording density receives attention.
In the conventional longitudinal magnetic recording type of disk drive, magnetization corresponding to recording data (binary data, 0 or 1) is formed in a longitudinal direction of a disk medium. On the other hand, in the perpendicular magnetic recording type of disk drive, the magnetization is formed in a depth direction of the disk medium.
Generally, the disk drive, data is recorded on the disk medium by a coding method of NRZ (non return to zero) recording. When the recording data is read from the disk medium with a head, a reproducing signal (read signal) becomes a dipulse signal string in the longitudinal magnetic recording method. On the other hand, in the perpendicular magnetic recording method, the reproducing signal becomes a pulse signal string including a low-frequency component of direct current (DC).
Generally, in a read channel (reproducing signal processing system including a read amplifier) of the disk drive, an analog front-end circuit of the read amplifier, an AC coupling, or the like has lower cut-off frequency characteristics. This is because a low-frequency noise component is removed from the reproducing signal to improve SNR (signal-to-noise ratio) of the reproducing signal.
In the perpendicular magnetic recording method, since the reproducing signal includes the low-frequency component, when the analog front-end circuit having the lower cut-off frequency characteristics cuts the low-frequency component, a phenomenon in which a base line of the reproducing signal is shifted is observed. When the shift in the base line of the reproducing signal occurs, a problem in which an error rate is increased occurs in decoding the recording data from the reproducing signal.
In order to improve the above-described problem, it is thought that the lower cut-off frequency of the read channel is decreased. However, when a passing band is simply broadened, the low-frequency noise component can not be cut, which results in degradation in SNR of the reproducing signal.
Since the read amplifier is particularly sensitive to the general low-frequency noise such as a 1/f noise, SNR is further degraded. Accordingly, when the lower cut-off frequency of the read channel is decreased in the perpendicular magnetic recording method, the error rate is inversely increased.
Conventionally, the method which can compensate the low-frequency component to cope with the shift in the base line has been proposed (For example, Jpn. Pat. Appln. KOKAI Publication No. 11-120702). However, it is difficult that the conventional method is applied to the perpendicular magnetic recording type of disk drive and the shift in the base line of the reproducing signal is efficiently eliminated.